The present invention relates to a switch circuit and a series regulator, and more particularly, to a switch circuit and a series regulator that selectively supplies power to circuits of portable electronic devices.
Recent portable electronic devices are provided with switch circuits to stop supplying power to circuits that do not need to be operated.
FIG. 1 is a block diagram showing a first example of a prior art electronic device 100, which includes a battery 12, a plurality of driven circuits 13 (only one shown) driven by the battery 12, and a plurality of switch circuits 11 (only one shown). Each switch circuit 11 is arranged in association with one of the driven circuits 13. The switch circuit 11 selectively supplies the associated driven circuit 13 with power in response to a control signal EN provided by a control circuit (not shown).
The switch circuit 11 includes an NMOS transistor 14, which functions as a main switch, a charge pump circuit 15, and a driver circuit 16. The transistor 14 has a drain connected to an input terminal IN of the switch circuit 11, a source connected to the output terminal OUT of the switch circuit 11, a gate connected to the driver circuit 16, and a back gate connected to the source.
The input terminal IN is supplied with a battery voltage VB. A control signal EN is provided to the charge pump circuit 15 and the driver circuit 16. When the control signal EN goes high, the charge pump circuit 15 pulls up the battery voltage VB to a predetermined voltage and supplies the driver circuit 16 with a drive voltage VD. The drive voltage VD is greater than the sum of the battery voltage VB and the voltage between the drain and gate of the transistor 14.
Further, the high control signal EN causes the driver circuit 16 to supply the gate of the transistor 14 with the drive voltage VD. This activates the transistor 14 and supplies the driven circuit 13 with the battery voltage VB.
When the control signal EN goes low, the charge pump circuit 15 is deactivated. Further, the driver circuit 16 connects the gate of the transistor 14 to the ground GND. This deactivates the transistor 14 and inhibits the supply of power to the driven circuit 13.
By selectively inhibiting the supply of power to the driven circuit 13 in this manner, the switch circuit 11 reduces the power supplied to the driven circuit 13 and thus, conserves battery power.
The transistor 14 has a parasitic diode 17 connected between the back gate and the drain. When the transistor 14 is deactivated, a back current flows through the parasitic diode 17 from the output terminal OUT toward the input terminal IN if the voltage VIN at the input terminal is less than a value obtained by subtracting the forward voltage VF of the parasitic diode 17 from the voltage VOUT at the output terminal OUT (VIN less than VOUTxe2x88x92VF).
Such a back current is prevented in a second example of a prior art switch circuit 21, which is illustrated in the block circuit diagram of FIG. 2. The back gate of the transistor 14 is connected to the ground GND in the switch circuit 21. The transistor 14 has a parasitic diode 22 connected between the back gate and the source and a parasitic diode 23 connected between the back gate and the drain. The parasitic diode 22 prevents back current from flowing from the output terminal OUT toward the input terminal IN.
However, in the switch circuit 21, the voltage at the back gate is at the ground level when the transistor 14 is activated. The on resistance of the transistor 14 thus increases when the input voltage increases, as shown in FIG. 3. In other words, the on resistance of the transistor 14 is affected by the input voltage. This significantly decreases the voltage at the switch circuit 21 and hinders the supply of sufficient power to the driven circuit 13.
To solve this problem, a third example of a prior art switch, which is illustrated in FIG. 4, is provided with two transistors 14a, 14b, which are connected in series and function as a main switch. However, the series connected transistors 14a, 14b increase the scale of the switch circuit.
A transistor 14 having a smaller resistance may be used to prevent the power supply voltage from decreasing. However, such transistor would occupy a relatively large space and also increase the scale of the switch circuit.
It is an object of the present invention to provide a switch circuit and a series regulator that is relatively small and has improved operational characteristics.
To achieve the above object, the present invention provides a switch circuit including an input terminal and an output terminal. The switch circuit includes a MOS transistor connected between the input and output terminals. A gate drive circuit is connected to a gate of the transistor to provide a gate drive signal thereto. The gate drive circuit responds to a first control signal provided to the switch circuit and causes the gate drive signal to have one of a first drive voltage, which is derived from a voltage at the input terminal, and a low potential power supply level. A back gate drive circuit is connected to a back gate of the transistor to provide a back gate drive signal thereto. The back gate drive circuit controls a voltage of the back gate drive signal in accordance with whether the transistor is activated or deactivated.
A series regulator includes an input terminal and an output terminal. The series regulator has a switch circuit including a MOS transistor connected between the input and output terminals. A gate drive circuit is connected to a gate of the transistor to provide a gate drive signal thereto. The gate drive circuit responds to a first control signal provided to the switch circuit and causes the gate drive signal to have one of a first drive voltage, which is derived from a voltage at the input terminal, and a low potential power supply level. A back gate drive circuit is connected to a back gate of the transistor to provide a back gate drive signal thereto. The back gate drive circuit controls a voltage of the back gate drive signal in accordance with whether the transistor is activated or deactivated. A comparison amplifier compares the voltage at the output terminal with a predetermined reference voltage to generate the first control signal based on the comparison result.
A switch circuit, which has an input terminal and an output terminal, includes a main switch connected between the input terminal and the output terminal, a charge pump connected to the input terminal and the main switch and receiving a first control signal. The first control signal activates and deactivates the charge pump. A driver circuit is connected to the charge pump and the main switch and receives the first control signal. When the first control signal is activated, the charge pump pulls up an input voltage supplied to the input terminal to a predetermined level and provides a drive voltage to the driver circuit, which in turn generates a drive signal having substantially the same voltage as the drive voltage. The driver circuit supplies the drive signal to the main switch, thereby activating the main switch such that the input voltage is supplied at the output terminal. When the first control signal is deactivated, the drive signal is deactivated, which deactivates the main switch such that the input voltage is not provided at the output terminal. A level shift circuit receives the first control signal and generates a second control signal having substantially the same voltage level as the input voltage. A back gate drive circuit is connected between the level shift circuit and the main switch and, in response to receiving the second control signal, the back gate drive circuit generates a back gate drive signal having substantially the same voltage as the voltage at the output terminal to the main switch.